Modify the logic for deleting variations

[gumgumcode]

Summary

Implements the following expected behavior which fixes an issue with Subscriptions:

1. Switch from variable subscription to non-variable one will delete variations.
2. Switch from variable subscription to variable product and vice-versa will NOT delete any variations.

This shall have no side-effects in WooCommerce Core.

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Closes #26959.
Closes https://github.com/woocommerce/woocommerce-subscriptions/issues/3672

How to test the changes in this Pull Request:

This was originally pushed to WC Subscriptions but it makes more sense to add it directly to WooCommerce. Testing instructions are therefore available in WC Subscriptions PR: https://github.com/woocommerce/woocommerce-subscriptions/pull/3795.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Improves the logic for deleting variations when a product type is changed

[rrennick]

@codestor4 This has the potential to change behaviour for third party product types. This could be implemented as a filter that defaulted to the current logic. WC Subs could use that filter to bypass removing the variations.

[gumgumcode]

Thanks for the feedback @rrennick. I have made the necessary changes now.

The newly introduced filters will be used as follows in https://github.com/woocommerce/woocommerce-subscriptions/pull/3732:

add_filter( 'woocommerce_from_product_type_changed', array( __CLASS__, 'maybe_keep_variations' ) );
add_filter( 'woocommerce_to_product_type_changed', array( __CLASS__, 'maybe_keep_variations' ) );

/**
* Prevent variations from being deleted when product type changes from variable product to another variable product.
*
* @since 3.0.8
*
* @param string $product_type The product type value to filter.
* @return string $product_type The filtered product type value.
*/
public static function maybe_keep_variations( $product_type ) {
if ( strpos( $product_type, 'variable' ) ) {
    $product_type = 'variable';
}
return $product_type;
}

I've tested this and it works. I'll be adding it to the pull request mentioned above after this current pull request gets merged.

[gumgumcode]

Update: I have removed the two filters and replaced it with one. It still achieves the same end goal.

[gumgumcode]

Hey @rrennick, no rush but could you review this once again when you have some time? Thx :)

[Konamiman]

Hi @codestor4, the change looks fine but it would be great if you could add a documentation comment for the newly added filter. See an example here: https://github.com/woocommerce/woocommerce/blob/master/includes/admin/meta-boxes/views/html-variation-admin.php#L165-L174 (please use version 4.9 for the @since field).

[gumgumcode]

@Konamiman I've made the requested changes. Please review and let me know! 👍

[gumgumcode]

@abhishek-pokhriyal It's ready for a review 👍
Note to self: never to use @ in commit messages, it appears I accidentally tagged a GH member.

[Konamiman]

Approved. @rrennick do you want to take a second look before merging?

[rrennick]

@codestor4 Sorry for the delays in getting this through review.